An X-ray CT apparatus reconstructs a cross-sectional image on the basis of measured projection data from various angles which are acquired by irradiating X rays from surroundings of an object, thereby displaying a cross-sectional image. A method of reconstructing a cross-sectional image based on the measured projection data is roughly classified into an analysis method and an iterative method. The analysis method is a method of analytically producing an image on the basis of the cut surface projection theorem, and is often employed in X-ray CT apparatuses since a cross-sectional image can be reconstructed from measured projection data with a small calculation amount.
The iterative method is a method of estimating a cross-sectional image through a repeated calculation on the basis of a model which is obtained by mathematically modeling an observation system for acquiring measured projection data or by statistically modeling fluctuation or the like in an amount of X rays included in the measured projection data. The iterative method is accompanied by repeated calculations and thus causes a calculation amount to be more than the analysis method, but can reduce or, artifact which comes from the analysis method, for example, a cone beam artifact, or quantum noise which comes from fluctuation in an amount of X rays.
The X-ray CT apparatus is widely used in medical sites as one of image diagnosis, but there are concerns about exposure to X rays. Particularly, in recent years, the attention to radiation exposure has been increased. For this reason, from the viewpoint of reduction in radiation exposure, the iterative method has attracted attention. The iterative method can reduce quantum noise even in measured projection data which is obtained with a low amount of rays. With the recent high performance of computers, the iterative method has been considered to be employed therein. For example, PTL 1 discloses an image processing apparatus which iteratively performs repeated calculations until an image measurement value within a region set on a cross-sectional image which is reconstructed using the analysis method reaches a desired value.